The present invention relates to a scanning optical system that is used as an optical system for a scanning optical device such as a laser beam printer.
The scanning optical device deflects a beam emitted from a light source such as a semiconductor laser by means of, for example, a polygonal mirror, and converges the beam to form a spot on a surface to be scanned such as a surface of a photoconductive drum, through an f.theta. lens (i.e., a scanning lens). The beam spot formed on the surface to be scanned moves (i.e., scans) on the surface in a predetermined scanning direction as the polygonal mirror rotates.
In such a scanning optical system, since an optical path length and an incident angle for each of the lens surfaces are different according to a scanning angle, transmitting light quantity, i.e., the quantity of light transmitted through the f.theta. lens varies as the laser beam is deflected. In general, the quantity of light transmitted through the optical elements between the polygonal mirror nd the photoconductive drum along the optical axis is larger than that through the peripheral portion of the elements. The variation of the quantity of the transmitted light due to change of the scanning angle is referred to as a power variation.
If the power variation exceeds the tolerance level, an exposure on the photoconductive dram varies widely, which reduces printing quality.